Prepared by: Douglas Daines

1. ECE5320 MechatronicsAssignment#1: Literature Survey on Sensors and Actuators Topic: Thermal Bimorph Actuators Prepared by: Douglas Daines 3/10/2006

2. Outline • Definition • Basic structure • Major applications • Basic working principle illustrated • Major applications • Design applications • Control • Model • Reference list • To probe further ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

3. Thermal Bimorph Actuators • Thermal bimorph actuators operate on the principle of thermal expansion. Two substrates are combined that have different expansion rates. When heated it results in bending of the material. • “Electro-thermal actuations are very attractive since they can generate large deflections and forces with low actuating voltages and their fabrication process is compatible with the general integrated circuit (IC) fabrication process.” An electro-thermal bimorph-based microactuator for precise track-positioning of optical disk drives J P Yang, X C Deng and T C Chong ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

4. Basic Structure • Thermal Bimorphs are generally etched into silicon • They are designed on the order of 10’s to100’s of micrometers • To facilitate even bending of a surface, the surfaces arms are kept very narrow • For a wide surface, the surface is etched into a comb structure MEMS actuators Professor Mark Bachman ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

5. Basic working principle illustrated • Voltage is applied to the actuator • The two parts of the actuator expand as they are heated • One side expands faster than the other resulting in curvature ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

6. Basic working principle illustrated • Many different types of material can be used • Metals • Semiconductors • Ceramics • Polymers ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

7. Major Applications • Optical displays • Biomedical imaging • Laser beam steering • Optical switching • Fast-scanning mirrors that can generate piston motion are required by applications for wave-front shaping in adaptive optics • Interferometry systems • Spatial light modulators • Tunable lenses for confocal microscopy • Microvalves • Actuators on magnetic recording heads • Precision micro positioning systems A thermal bimorph micromirror with large bi-directional and vertical actuation Ankur Jain, Hongwei Qu, Shane Todd, Huikai Xie ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

8. Thermal Bimorph Micromirror • The bimorph actuation structure is composed of aluminum and silicon dioxide with an embedded polysilicon thermal resistor. • With a size of only 0.7mm×0.32 mm, the micromirror demonstrated a vertical displacement of 0.2mm at 6V dc. A thermal bimorph micromirror with large bi-directional and vertical actuation Ankur Jain, Hongwei Qu, Shane Todd, Huikai Xie ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

9. Thermal Bimorph Micromirror “This device can be used to perform bi-directional rotational scanning through the use of two bimorph actuators. The micromirror rotates over ±15◦ at less than 6V dc, and over ±43◦ (i.e., >170◦ optical scan angle) at its resonant frequency of 2.6 kHz.” ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

10. Thermal Bimorph Micromirror • Limitations • Small displacements • ~200µm for a device 700µm×320µm in size. • Designed for precision systems • Very sensitive to external temperature • Benefits • The fabrication process is simple and compatible with CMOS processes. Because of this, control circuitry can be integrated with the mirror on the same chip. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

11. Dip Pen Nanolithography (DPN) • “In Dip Pen Nanolithography (DPN), arbitrary nanoscale chemical patterns can be created by the diffusion of chemicals from the tip of an atomic force microscope (AFM) probe to a surface.” • The probe array is 10 thermal bimorph probes made of silicon nitride and gold. The probes are 300 µm long and the tips are spaced at 100 µm intervals. Design, Fabrication, and Characterization of Thermally Actuated Probe Arrays for Dip Pen Nanolithography David Bullen, Xuefeng Wang, Jun Zou, Member, IEEE, Sung-Wook Chung, Chad A. Mirkin, and Chang Liu ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

12. DPN – AFM Probes 10 probe DPN array. The inset shows a close-up of the tip • Each probe is a thermal actuator, enabling it to lift itself from the surface • The actuator moves the probe tip up 8µm with a current of 10mA ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

13. DPN – AFM Probes “Each probe is made of silicon nitride with a molded pyramid tip at its free end. The addition of a gold layer on the tip side of the cantilever creates a bimorph actuator. “Thermal bimorph actuation was chosen in lieu of other actuation methods because of its small footprint, large deflection, low voltage operation, and ease of fabrication. “The heat supplied by the resistive heater is treated as a function of temperature due the relatively large temperature coefficient of resistance of the heater material.” ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

14. DPN • The array is demonstrated by using it to simultaneously write 10 unique octadecanethiol patterns on a gold surface. Individual pictures are 8µm x 8µm ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

15. AFM Probes • Limitations • Contact forces can easily damage probe • Thermal design of the probe is complex • Benefits • Small size • Large deflection • Low voltage • Ease of fabrication ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

16. Control of Thermal Bimorphs • Thermal Bimorphs are a popular micro actuator in MEMS (Micro Electrical Mechanical Systems) • “In most cases, closed-loop feedback control is not feasible for these devices due to the difficulty in integrating on-wafer position sensors. Capacitive position sensing can be used, but it requires fairly complex electronics, due to the very low capacitance ranges.” • More precise modeling is used to try and design systems with better control Dynamic Modeling and Input Shaping of Thermal Bimorph MEMS Actuators Dan 0. Popat, Byoung Hun Kangt, John T. Went, Harry E. Stephanout, George Skidmore, Aaron Geisberged ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

17. Control of Thermal Bimorphs • When thermal bimorph actuators are used with other MEMS devices, it is desirable to minimize vibrations • Input shaping is a popular way to address vibration • Normal control procedures for open loop systems apply. • Settling times for these actuators can be in the range of a few milliseconds • Modeling of these actuators requires a high degree of analysis • Thermal analysis is required to develop models in addition to mechanical analysis ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

18. Model of Thermal Bimorphs Resulting equations with three states (x, Thot, Tcold), one input (0, and one output (x): By eliminating the temperatures state in the discretized equation through back-substitution, and further assuming that the g coefficients are negligible, we obtain the following 3rd order ARX model: • Current applied to heating pads results in thermal expansion • Resistance is temperature dependant • Mass, damping, and stiffness need to be taken into account ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

19. To Probe Further • IEEE Explore – Journals http://ieeexplore.ieee.org/ • University of Birmingham - Micro-Engineering and Nano-Technology Research Centre http://www.micro-nano.bham.ac.uk/index.htm • The NASA Astrophysics Data System • http://adsabs.harvard.edu/ ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

20. Reference list • A thermal bimorph micromirror with large bi-directional and vertical actuation Ankur Jain, Hongwei Qu, Shane Todd, Huikai Xie • An electro-thermal bimorph-based microactuator for precise track-positioning of optical disk drives J P Yang, X C Deng and T C Chong • MEMS actuators Professor Mark Bachman • Dynamic Modeling and Input Shaping of Thermal Bimorph MEMS Actuators Dan 0. Popat, Byoung Hun Kangt, John T. Went, Harry E. Stephanout, George Skidmore, Aaron Geisberged • Design, Fabrication, and Characterization of Thermally Actuated Probe Arrays for Dip Pen Nanolithography David Bullen, Xuefeng Wang, Jun Zou, Member, IEEE, Sung-Wook Chung, Chad A. Mirkin, and Chang Liu ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators